<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        var age = parseInt(prompt('请输入你的年龄')); // 18
        // alert(typeof age); string

        //不管用户输入的是 100 'hello' true undefined null  {"name":"哈哈"}
        // age的类型一定是字符串 
        document.write('再过五年你的年龄就是:'+(age+5))
        //再过五年你的年龄就是:185


    </script>
</body>
</html>